The invention further relates to a method of checking the transmission properties of a volume tomograph using a test body in which a series of a plurality of measurement recordings of the test body is produced with the volume tomograph and a stack comprising a plurality of layer images of the test body is calculated from the series by a tomography algorithm or rear-projection algorithm in the volume tomograph itself or in an evaluation unit.
Within the meaning of the invention described here, tomographs, in particular volume tomographs, are understood to mean all devices with which it is possible to calculate a stack comprising a plurality of layer images of a body under examination from a series of recorded measurement values, in particular from a series of recorded images, with the assistance of the tomography algorithm or rear-projection algorithm.
At the same time, for the term volume tomograph used here, it does not matter as far as the invention is concerned which measuring principle is used as a basis for recording the individual measured values or, in the special case, for recording the images (for example in the case of an X-ray computer tomograph or a magnetic resonance tomograph).
For example, volume tomographs can replace X-rays to calculate a stack of layer images from a series of individual X-ray images, for example such images that each represent a two-dimensional projection of the X-ray absorption, after the images have been recorded, such a stack representing a three-dimensional image of the body under examination. Accordingly, in such a volume tomograph, use is made of 2D image sensors whose sensor pixels record the measured values.
Also known are magnetic resonance tomographs or magnetic resonance volume tomographs, with which so-called spin flips that lead to the transmission of electromagnetic waves that can be received by antennas and are characteristic of the material to be examined, can be produced by the action of external magnetic fields in a tissue to be examined. Here, the volume range (voxel) of the body under examination from which the received electromagnetic wave originates is determined by the superimposition of magnetic fields. These measured values can also be converted to layer images by tomography or rear-projection algorithms.
In sonography, i.e. in ultrasound examinations, it is also possible to create sonographic recordings that are based on the reflection of ultrasonic waves. Examples of further known tomographic devices are Digital Volume Tomographs (DVT) and also the following devices: Cone Beam Computer Tomographs (CBCT) or tomographs that are used to record the volume information in extremities, or devices for the spatial representation of the female breast (tomosynthesis).
For the test body, and in particular for the method according to the invention, it does not matter which kind of tomograph, preferably volume tomograph, is used, as the method relates to the evaluation of the layer images created or 3D recordings resulting therefrom.
In order to be able to draw definite conclusions relating to the particular body under examination, such as for example the different kinds of tissue or the types of material in general of the body and their dimensions, from tomographic recordings created by such above-described devices, it is necessary for the transmission properties of these volume tomographs to fulfill certain quality criteria.
As an example, the so-called Modulation Transfer Function (MTF) or contrast resolution is therefore an important quantity, based on which a conclusion can be drawn relating to the quality with which different types of tissue or materials in general of a body under examination can differ from one another, such as for example healthy tissue and tumorous tissue.
Furthermore, it is known that volume tomographs and the in particular manufacturer-specific algorithms used, like the above-described rear-projection algorithms, can lead to geometric distortions in the visual reproduction of a body under examination compared with the actual dimensions of the body. Such distortions or too inaccurate spatial resolution cannot be tolerated, especially in prosthodontics or even in radiation therapy, as here the target volume must be determined as accurately as possible in order to avoid damage to surrounding tissue. Knowledge of such distortions is also necessary, for example, when visual reproductions of the same body from different tomography systems are to be superimposed (matched).
Examples of further quality criteria are the signal-noise ratio and also the noise-power spectrum.
In the prior art, it is known to provide test bodies with which it is possible to check volume tomographs with regard to the above-mentioned criteria or transmission properties, such as for example contrast and spatial resolution. Particularly in the radiologically based planning of interventions, for example in prosthodontics or radiation therapy, the uncertainty of the position determination in the target volume must be kept as small as possible in order to protect healthy tissue.
Such test bodies are therefore used in particular to enable the contrast and spatial resolution of the volume tomographs to be tested. In so doing, it is known that such test bodies comprise structural elements that can be differentiated from the surroundings by a volume tomograph in the spatial 3D representation or the individual layer images of a layer image stack. With regard to their application in radiology, such test bodies are also referred to as radiological phantoms or as test bodies in general.